A voltage controlled oscillator (VCO) is an oscillating circuit that produces an output having a frequency that can be controlled by a DC voltage input to the VCO. VCO's are commonly used in phase-locked loops in communications equipment including but not limited to wireless devices such as cellular telephones. Conventionally, VCO's have been implemented with on-chip inductors and capacitors (LC) or with piezoelectric micro-electro-mechanical systems (MEMS) resonators, including film bulk acoustic resonators (FBAR's) and aluminum nitride (AlN) contour mode resonators (CMR's).
On-chip LC VCO's have a wide tuning range, greater than 10%, for example. Because on-chip inductors generally have a low Q-factor (generally <30), such VCO's are generally characterized by high phase noise. This may be disadvantageous in certain environments where low phase noise and/or small size are desirable. MEMS-based VCO's, on the other hand, may be significantly smaller due to the small form factor of MEMS resonators. Furthermore, the high Q-factor (>1000) of these resonators results in a VCO that exhibits low phase noise. However, due to the low electromechanical coupling of these mechanical resonators, MEMS VCO's generally have a small tuning range, less than 3%, for example. This limited tuning range makes it difficult to cover large portions of a signal band without using a prohibitively large number of resonators.
It would therefore be desirable to provide a VCO that could be configured to cover a relatively large frequency range, the 100 MHz to 3 GHz range of cellular frequencies, for example, that is also compact in size with a small number of resonators and that exhibits low phase noise.